The major focus of this research has been the pattern of expression of rabbit class II major histocompatibility antigens. The genes encoding the Alpha chains of these antigens have been recently cloned in this laboratory and is described in another section of this report. Probes derived from the cloned rabbit class II MHC antigen genes and human class II probes have been used to examine mRNA prepared from normal rabbit tissues and a number of newly established class II-positive in vitro propagated cell lines. The first objective is to determine which genes are normally expressed and in what cell types the expression occurs. Using the DCAlpha probe, mRNA from spleen and appendix were strongly positive, as expected for rich sources of B lymphocytes. Lymph node was positive at a lower level as were lung and testes. The result with testes is undergoing closer examination to determine the cellular source of the signal. Class II antigen-positive cell lines have been prepared by viral transformation of rabbit cells with several oncogenic DNA viruses. In addition, a class II-positive cell line has been derived from a spontaneous rabbit breast adenocarcinoma. These cell lines have been shown to contain mRNA transcripts reactive with human class II probes and are being probed now to determine their expression of each of the cloned rabbit class II Alpha chains.